Not applicable
1. Field of the Invention
The present invention is directed to a method for the production of hydrogen gas. More particularly, the present invention is directed to a method for the production of hydrogen gas by steam reduction wherein steam is contacted with molten metal to form a metal oxide and a hydrogen-containing gas stream. The metal oxide can then be reduced back to the metal for further production of hydrogen gas. The hydrogen gas can be used for the generation of energy and in various chemical processes, such as the treatment of coal and the production of ammonia.
2. Description of Related Art
Hydrogen (H2) is a valuable commodity and specialty chemical that is critical to a number of industrial processes including the production of ammonia and the refining of oil. In addition, hydrogen can be converted directly to electricity in fuel cells at efficiencies approaching 80 percent. Water is the sole by-product of hydrogen conversion in fuel cells, and toxic emissions are eliminated. For these reasons, hydrogen is widely considered the fuel of the future.
It is known that hydrogen gas can be produced from many different feedstocks such as natural gas, biomass or water using different techniques such as reformation, gasification or electrolysis. The most common methods are steam methane reformation (SMR), coal gasification, steam reduction, biomass gasification and pyrolysis, and electrolysis.
Steam methane reformation is believed to be the most economical and commercially viable process that is presently available. In the SMR process, methane (CH4) is reacted with steam (H2O) to form a gas stream that includes hydrogen and carbon monoxide (CO). The feedstock is typically natural gas and the cost of the natural gas represents a significant portion of the total production cost.
At least two major difficulties are associated with the SMR method. One difficulty is the dependency of the hydrogen production cost on the price of natural gas. The price of natural gas is highly volatile due to supply/demand issues, which are projected to persist into the future. Secondly, hydrogen produced by SMR is co-mingled with a significant quantity of carbon oxides that can only be partially removed by scrubbing or pressure swing adsorption, both of which are costly. The carbon oxides remaining in SMR hydrogen are detrimental to the catalysts employed in fuel cells and in the production of ammonia (NH3) from hydrogen.
Hydrogen production by coal gasification is another established commercial technology, but is only economically competitive when natural gas is prohibitively expensive. In the coal gasification process, steam and oxygen (O2) are utilized in the coal gasifier to produce a hydrogen-containing gas. High purity hydrogen can then be extracted from the synthesis gas by a water-gas shift reaction followed by removal of the carbon dioxide (CO2) by pressure swing adsorption or scrubbing. Impurities such as acid gases must also be separated from the hydrogen. Hydrogen can also be formed by the gasification of other hydrocarbons such as residual oil.
The steam reduction method utilizes the oxidation of a metal to strip oxygen from steam (i.e., steam reduction), thereby forming hydrogen gas. This reaction is illustrated by Equation 1.
xMe+yH2Oxe2x86x92MexOy+yH2xe2x80x83xe2x80x83(1)
To complete the cycle in a two-step steam reduction process, the metal oxide must be reduced back to the metal using a reductant. For example, carbon monoxide (CO) has an oxygen affinity that is similar to the oxygen affinity of hydrogen and they are equal at about 812xc2x0 C. At temperatures above about 812xc2x0 C., CO has a greater affinity for oxygen than does hydrogen. Thus, if the CO has an oxygen affinity greater than the oxygen affinity of the metal at equilibrium, the CO will reduce the oxide of Equation 1 back to the metal.
MexOy+yCOxe2x86x92xMe+yCO2xe2x80x83xe2x80x83(2)
Generally stated, the function of the metal/metal oxide couple is to transfer oxygen from the steam to the reducing gas (CO) without allowing the steam/hydrogen of the hydrogen production step to contact the carbon monoxide/carbon dioxide of the metal oxide reduction step. The metal and metal oxide are not consumed by the overall process.
Oxygen partial pressure (pO2) relates to the facility with which the metal may be oxidized (e.g., by steam) and the oxide may be reduced (e.g., by CO). A related mathematical expression is pH2O/pH2, which is proportional to the oxygen partial pressure. Also, an equivalent and inversely related quantity is the hydrogen fraction, expressed as:                               pH          2                          (                                    pH              2                        +                                          pH                2                            ⁢              O                                )                                    (        3        )            
Certain metals react strongly with water, releasing hydrogen. Examples of such metals include: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), silicon (Si), phosphorus, (P), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), yttrium (Y), zirconium (Zr), niobium (Nb), lanthanum (La), hafnium (Hf), tantalum (Ta) and gallium (Ga). The oxygen partial pressure in equilibrium with these metals and their oxides together is extremely low. Once the oxides are formed, they cannot be effectively reduced back to the metal by carbon monoxide. Conversely, there is another group of metals that produce insignificant quantities of hydrogen when reacted with water. Examples of these metals include: nickel (Ni), copper (Cu), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), mercury, (Hg), lead (Pb), bismuth (Bi), selenium (Se) and tellurium (Te). The oxygen partial pressure in equilibrium with these metals and their oxides together is quite high. The oxides, therefore, can be easily reduced by carbon monoxide.
Between the two foregoing groups of metals are other metals characterized by an oxygen affinity that is roughly the same as the oxygen affinity of hydrogen. Included in this intermediate group of metals are: germanium (Ge), iron (Fe), zinc (Zn), tungsten (W), molybdenum (Mo), indium (In), tin (Sn), cobalt (Co) and antimony (Sb). These are elements that readily produce hydrogen from steam wherein the resulting oxide can be reduced by carbon monoxide. That is, these metals have an oxygen affinity such that their equilibrium pH2O/pH2 is low enough to be practical for the production of hydrogen, yet the metal oxide is readily reduced by carbon at normal pyrometallurgical temperatures (e.g., about 1200xc2x0 C.). These metals are referred to herein as reactive metals, meaning that both the metal can be oxidized by steam and the metal oxide can be reduced by carbon monoxide.
The steam reduction/iron oxidation process was the primary industrial method for manufacturing hydrogen during the 19th and early 20th centuries. At elevated temperatures, iron strips oxygen from water, leaving pure hydrogen.
3Fe+4H2Oxe2x86x92Fe3O4+4H2xe2x80x83xe2x80x83(4) 
Excess water is required to maximize hydrogen production from a given amount of iron. After the hydrogen is produced, excess water is condensed leaving an uncontaminated hydrogen gas steam. The reaction products of the steam reduction/iron oxidation process are pure hydrogen and wustite (FeO) and/or magnetite (Fe3O4). To regenerate the metal, carbon monoxide or carbon captures the oxygen from the iron oxide, forming iron metal and carbon monoxide or carbon dioxide (CO2).
In the early years of hydrogen manufacturing, these two steps were carried out at different locations. The primary cost components for producing hydrogen by this method included the cost of the iron used minus the value received for the iron oxide produced and the cost of producing the excess steam required to drive the reaction (a function of temperature). This cost was reduced by the benefits derived from recovering excess energy from the steam. There are numerous examples in the prior art of the foregoing method.
U.S. Pat. No. 1,345,905 by Abbott discloses the production of hydrogen from steam by oxidation of iron using multiple reactors. In a four-reactor configuration, one reactor is used for iron oxidation (hydrogen production), two are used for iron oxide reduction and the fourth is used for preheating the reactants. Gas flows can be switched among the reactors for the continuous production of hydrogen.
U.S. Pat. No. 4,343,624 by Belke et al. discloses a 3-stage hydrogen production method and apparatus utilizing steam reduction as the hydrogen source. In a first stage, a low BTU gas containing hydrogen and carbon monoxide is formed from a feedstock such as coal. The low BTU gas is then reacted in a second stage with magnetite to form iron, carbon dioxide and steam. The steam and iron are then reacted in a third stage to form hydrogen gas and magnetite. It is disclosed that the magnetite can be returned to the second stage for use in the reduction reaction, such as by continuously returning the magnetite to the second stage reactor via a feed conduit. At least one of the stages takes place in a rotating fluidized bed reactor.
U.S. Pat. No. 4,555,249 by Leas discloses a gas fractionating unit that contains a reagent powder, such as an iron alloy, having a significant weight difference between the reduced form and the oxidized form. The unit includes two zones for containing the reagent powder, an oxidation zone and a reduction zone, wherein hydrogen gas is extracted from the oxidation zone. As the reagent powder is converted from the oxidized to the reduced form, the weight of the powder increases and the change in weight is utilized to transfer the reduced powder to the oxidation zone while moving the oxidized powder to the reduction zone.
The article xe2x80x9cH2 from Biosyngas via Iron Reduction and Oxidationxe2x80x9d, by Straus et al., discloses a method for hydrogen production from biosyngas. The biosyngas, which includes H2, CO, H2O, and CO2 with traces of N2 and CH4, is used to reduce magnetite to iron. The iron is then cooled and fed to a hydrogen gas generator where the iron is contacted with steam to form hydrogen by steam reduction. The iron oxide is then cooled and returned to the metal oxide reduction reactor for reaction with the biosyngas.
Disadvantages of the steam reduction process utilizing iron include that the reaction of solid iron with steam produces an oxide layer, which inhibits additional steam from reacting with iron beneath the oxide layer and therefore the rate of reaction is limited by the rate of gas-diffusion through the oxide layer. Also, the rate of reaction is dependent upon the surface area of the iron available for the reaction. High surface area, however, is equated with small particle size, and small particles are expensive to process. Further, there are difficulties associated with reducing the iron oxide. One method for reducing the iron oxide includes smelting of the oxide and therefore carries the disadvantage of high temperature due to the high melting point of iron (1538xc2x0 C.). In another method, the iron oxide is reduced to the metal in the solid state by carbon and/or reducing gas. This latter process, however, is inefficient and kinetically difficult. Excluding reduction in the solid state, for each ton of hydrogen produced, a minimum of 20.8 tons of iron and 28.7 tons of magnetite must physically be moved from one reactor (metal oxidation) to another (metal reduction).
Other metals besides iron have been used for steam reduction processes. U.S. Pat. No. 1,050,902 by Acker discloses the use of tin or zinc in a steam reduction process to form hydrogen and the regeneration of the metal by reduction of the metal oxide with coal.
U.S. Pat. No. 3,821,362 by Spacil illustrates the use of Sn/SnO2 to form hydrogen. Molten tin is atomized and contacted with steam to form SnO2 and hydrogen gas. The SnO2 is then contacted with a producer gas composed of H2, N2 and CO, which is formed by contacting powdered coal with air. The SnO2 is reduced to liquid tin, which is then transferred back to the first reactor. A similar method for hydrogen production is illustrated in U.S. Pat. No. 3,979,505 by Seitzer.
U.S. Pat. Nos. 4,310,503 and 4,216,199 by Erickson disclose a comprehensive investigation of the potential of tin to act as the carrier of the oxygen from steam to carbon dioxide. An extension of this work is also reported in xe2x80x9cHydrogen from Coal via Tin Redoxxe2x80x9d by Erickson, prepared for the Office of Energy Related Inventions, U.S. Department of Energy (February 1981).
Erickson reports that the yield of hydrogen obtainable from a given quantity of carbonaceous reducing composition can be increased by using a multi-staged process wherein successive stages are arranged in order of increasing equilibrium pH2/pH2O for the metal oxidation reaction with steam. Among the substances used as intermediates (i.e., metal/metal-oxide couples) were pure solids such as iron, wustite (FeO), tungsten dioxide (WO2), molybdenum and germanium; pure liquids such as tin and indium; and dissolved liquids such as tin, indium, germanium, zinc and iron, where dissolved means that the intermediate is present at less than unit activity. The effect of employing a dissolved liquid is that the oxygen partial pressure is increased (i.e., the hydrogen fraction is decreased) and less hydrogen is produced. Erickson discloses that suitable solvents for the dissolved liquids may be selected from one or more of the non-reactive metals that have a high oxygen partial pressure, for example copper, lead and nickel. It is also disclosed that tin, a reactive metal, can serve as a solvent for indium.
When forming hydrogen by using tin in a steam reduction process, the first reaction is:
Sn+2H2Oxe2x86x92SnO2+2H2xe2x80x83xe2x80x83(5) 
To secure reasonable kinetics for the above reaction, a temperature above about 900xc2x0 C. is required, and tin is a liquid at that temperature (Tm=232xc2x0 C.). At such high temperatures thermodynamics dictate that a large excess of steam is required in order for the reaction to proceed. The need for a large excess of steam creates a number of problems. Heat must be recovered for the process to be economical, including the heat of evaporation of the water to form the steam. Technically, most of this heat recovery is possible, although doing so requires additional capital. Also, a large excess of steam must physically contact the tin, such as by being bubbled through the liquid tin. Practically, such contact is possible only if the steam and reactor are operated under considerable pressure and, generally, processes that operate at very high pressures are quite expensive.
For example, the production of one ton of hydrogen requires the reaction of 8.94 tons of steam with 29.4 tons of tin (stoichiometric calculations). Additionally, the production of one ton of hydrogen at 900xc2x0 C. requires 35.7 tons of steam to satisfy the thermodynamic requirement. If this total steam requirement (44.6 tons) is passed through the stoichiometric quantity of tin at atmospheric pressure, the velocity of the steam through the space that otherwise would be occupied by the tin must be in excess of 100 meters per second. This yields a nominal residence time of less than {fraction (1/100)} of a second. Even if system pressure is raised to 100 atmospheres, only 0.85 seconds are available for the reaction to approach equilibrium. An amount of tin in excess of the stoichiometric requirement can be used, and the effect of a larger weight (larger volume) of tin is to increase the nominal residence time. This approach of increasing nominal residence time is expensive, however, because of the increased size of the tin-steam reactor and increased inventory of tin required.
Thus, there remains a need for a method for producing hydrogen that is technically sound and economically viable. Both the steam/iron and steam/tin processes are technically viable. Neither, however, meets the requirement for economic viability. The steam/iron process is not satisfactory because: (1) the production of hydrogen is diffusion controlled; (2) the cost of moving the metal is high; and (3) the difficulty (cost) of reducing the iron is high. The steam/tin process fails the economic viability requirement because of poor kinetics at low temperatures (below about 800xc2x0 C.) and poor thermodynamics at higher temperatures. The consequence of poor thermodynamics is the requirement that large amounts of steam be processed through the molten metal, which increases the difficulty and cost of the process. Due to these and other factors, the present inventors are not aware of a commercial facility that is practicing the steam reduction method, despite the high demand for hydrogen gas.
According to one embodiment of the present invention, a method for the production of a hydrogen-containing gas stream is provided. The method includes the steps of generating steam and contacting the steam with a molten metal mixture having at least about 20 weight percent iron dissolved in a diluent metal, wherein at least a portion of the iron is oxidized to a metal oxide and at least a portion of the steam is reduced to form a hydrogen-containing gas stream. Preferably, the diluent metal is tin. By contacting the steam with iron that is dissolved in a diluent metal, the problems associated with thermodynamic and kinetic limitations inherent in prior steam reduction methods are reduced.
According to another embodiment, a method for the production of a hydrogen-containing gas is provided wherein steam is generated and is contacted with a molten metal mixture at a temperature of at least about 1100xc2x0 C. The molten metal mixture includes a reactive metal dissolved in a diluent metal wherein the reactive metal is oxidized and the steam is reduced to form hydrogen. The use of temperatures of at least about 1100xc2x0 C. for the molten metal mixture enables the production of hydrogen under favorable thermodynamic and kinetic conditions.
According to another embodiment, a method for the production of a hydrogen-containing gas stream is provided that includes the steps of generating steam, contacting the steam with a molten metal mixture in a reactor, wherein reactive metal-contaning particles are dispersed in the molten metal mixture. The reactive-metal containing particles advantageously supply additional reactive metal to the molten metal mixture as the reactive metal is oxidized by the steam.
According to another embodiment, a method for the production of a hydrogen-containing gas stream is provided that includes the steps of contacting steam with a molten metal mixture in a reactor, the molten metal mixture including a reactive metal dissolved in a diluent metal. The reactive metal is oxidized to a metal oxide by the steam. The metal oxide is then reduced back to the reactive metal within the reactor.
The hydrogen-containing gas stream produced in according with the foregoing can be used in a variety of processes and is particularly suited to the treatment of carbon-bearing substances such as coal or waste products and to the manufacture of chemicals such as ammonia.